Yasuyuki Shimura

A Yb-based intermetallic compound YbCo2 exhibiting a magnetic-field-induced order is known to show a giant specific heat divided by temperature, C/T∼ 6.5 J/K2 mol, around 0.3 K. We investigate the potential of this substance as the magnetic refrigerants by measuring the thermodynamic properties down to 0.1 K. The entropy change per volume by applying magnetic field of 3 T is found to be −ΔSM∼ 0.15 J/K cm3 around 1 K. This value is particularly large in the Yb-based metallic magnetic refrigerants to provide the sub-Kelvin temperature. In addition, we demonstrate that a 2.2 g sample is cooled down to 0.26 K by the adiabatic demagnetization refrigeration from 1.8 K and 12 T. This cooling performance is attributed to the ground state, at zero field, locating around the boundary between the field-induced ordered phase and the paramagnetic one. Our study clarifies the utility of the sub-Kelvin magnetic refrigerants, which combine the large −ΔSM and the high thermal conductivity of metal.

Magnetic entropy remaining near absolute zero temperature in the antiferromagnetic (AFM) critical state is expected to be usable for the cryogenic magnetic refrigeration. In this paper, we report quasi-adiabatic demagnetization refrigeration for Ce2(Cu1−xNix)2In (0≤x≤0.38) whose parent compound Ce2Cu2In orders antiferromagnetically below 6 K, by using a laboratory-made cell attachable to a commercial 4He refrigerator with a base temperature of 1.8 K. The final temperature by the quasi-adiabatic demagnetization as a function of x indicates a local minimum of 1 K at the AFM critical composition of x∼0.3. The validity of this result is confirmed by our specific-heat measurements. Our work motivates the future developments of Ce-based cryogenic magnetic refrigerants with the lower characteristic temperature in view of the low cost and easiness in synthesizing compared with Yb-based counterparts.